FORTUNE — Dear Annie: Since my department was shrunk by layoffs a couple of years ago, I have gotten an in-house reputation as a champion multitasker, able to handle half a dozen projects at one time (and leap tall buildings in a single bound). Working this way is not only exhausting, but I think it makes me less competent. Everything takes much longer than it would if I could just focus on one task at a time.
So I made a New Year’s resolution to cut it out, or at least do less multitasking. The problem is my boss, who is constantly piling more stuff on me to do “with my left hand,” as he puts it, when I’m already busier than a woodpecker in a lumberyard. Can you or your readers suggest any way to convince him that we’d all be better off tackling one thing at a time? — Frazzled
Dear Frazzled: A heap of academic research has demonstrated conclusively that the human brain needs time to refocus every time we turn our attention in a different direction. A famous experiment at Stanford in 2009, for instance, found that multitaskers are “more susceptible to interference from irrelevant environmental stimuli” — that is, more easily distracted and less efficient at what they’re trying to do.
So your impression that doing several things at once slows you down and trips up your productivity is correct. But if your boss isn’t impressed with academic research, Sanjeev Gupta, CEO of consulting firm Realization, suggests a quick game to prove the point.
Here’s how it works: Get a stopwatch, or any watch with a second hand, and time how long it takes you (or your boss) to write “multitask 123456789.” Next, time how many seconds it takes to write the same thing, but this time with the numbers interspersed between the letters: “m1u2l3t4i5t6a7s8k9.”
Simple, right? When I tried this, it took me 9.5 seconds to write “multitask 123456789” — and 24 seconds, or more than twice as long, to write the version that requires switching back and forth from letters to numbers.
Okay, so maybe I’m just lousy at multitasking but, says Gupta, even master multitaskers (like you, perhaps) will see a dramatic difference. “It seems counter-intuitive, but if you give two employees two identical tasks, the one who focuses on one task at a time will always finish faster and with fewer errors than the one who multitasks,” Gupta says.
“Many people boast about their multitasking prowess on their resumes and in job interviews,” he adds. “But in reality, this is like saying, ‘I don’t get things done as quickly, or as well, as others do.’”
Gupta can back up his views on this with dollars-and-cents evidence. For the past 10 years, his company has been helping clients rid their projects of multitasking, which has boosted productivity at outfits like Siemens (SI), Boeing (BA), Delta, and the U.S. Air Force by 50% or more, adding an estimated total of $3.5 billion in extra cash and profits.
A couple of examples: By redesigning processes to eliminate multitasking, medical software maker Medtronic (MDT) cut its new-product release time from nine months to two months. At Boeing, cutting out multitasking boosted productivity on the manufacture of satellites by 64%.
“There is no question that concentrating on one thing at a time will make you more productive and give you a real competitive advantage,” Gupta says. Fine, but in practical terms, how do you keep your resolution to cut down on multitasking?
“First, take a look at why you are multitasking in the first place,” he suggests. In the companies where Realizations does its consulting work, Gupta sees two main causes of doing several things at once.
“The first one is, you interrupt yourself in the middle of a task because something is missing, so you aren’t prepared to finish it,” he says. “So you drop it and do something else.”
The remedy for that: Analyze the task in advance and make sure before you start that you have everything you’ll need to carry it through to the end.
The second chief cause — and the one that seems to apply to your situation — is that “your boss lacks clear priorities, so he expects you to multitask to make up for that.”
Fixing this will take some diplomacy on your part. The next time he throws you something else to do when you’re already overloaded, “sit down with him and ask him to rank the various assignments in order of priority,” Gupta advises. “Which are the most important things to get done immediately, and which ones can wait a bit?”
Of course, you may well be able to determine this for yourself without asking, in which case you’ll have to get used to setting your own priorities — and then sticking with them. This will take some practice. Multitasking can, after all, be as hard a habit to break as any other. But, says Gupta, “you will find that your increased productivity, and better quality of work, is worth the effort.” Good luck.
Talkback: Do you consider yourself a proficient multitasker? Do you find that doing several things at once makes you more productive, or less so? Leave a comment below.